a) Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing same, and more particularly, to a semiconductor device handling a plurality of voltage levels, and a method of manufacturing same.
b) Description of the Related Art
Drive voltages for semiconductor integrated circuits have tended to fall as savings in power consumption are made. For example, analogue circuits, such as a TTL, have been developed with a 5V power source, but subsequently developed digital circuits are generally driven at 3.3 V. The drive power supply for digital circuits has been tending to fall even further, to 2.5V. In memory devices, and the like, even more significant voltage reductions have been achieved. The signal level depends on the power supply voltage, such that, for example, the level is “1” when the voltage is 70% or above of the power supply voltage and it is “0” when the voltage is 30% or less.
Integrated circuit devices containing several types of functional circuit are sometimes required to handle a plurality of voltage levels. For the sake of convenience, such devices are known as multi-voltage level devices. These contain a high-voltage circuit driven by a relatively high-voltage power supply and a low-voltage circuit driven by a relatively low-voltage power supply.
In a metal-oxide-semiconductor (MOS) transistor, the source electrode is usually grounded and the power supply voltage is supplied to the drain electrode. The gate electrode can be supplied optionally with a ground potential or the power supply voltage. The gate oxide film must have a breakdown voltage corresponding to the power supply voltage in the vicinity of the source electrode and the drain electrode. In a multi-voltage level device, the low-voltage circuit and the high-voltage circuit have different voltage levels supplied to the gate electrode.
In certain types of multi-voltage level device, the gate oxide film differs in thickness in low-voltage circuit MOS transistors and high-voltage circuit MOS transistors. Low-voltage circuit MOS transistors have a thin gate oxide film and high-voltage circuit MOS transistors have a thick gate oxide film. By varying the thickness of the gate oxide film in this way depending on the power supply voltage, it is possible to use high-performance MOS transistors for both low-voltage circuits and high-voltage circuits. However, since the gate oxide films vary in thickness, it is necessary to provide separate gate oxide film forming processes. This increases the number of manufacturing steps, and hence causes manufacturing costs to increase.
If low-voltage circuits and high-voltage circuits are fabricated using gate oxide films of uniform thickness, then MOS transistors in low-voltage circuits will have the same thickness as MOS transistors in high-voltage circuits. A suitable thickness for high-voltage circuit gate oxide films will be unnecessarily thick for low-voltage circuits, and hence MOS transistor performance will decline.
As described above, if a low-voltage circuit and high-voltage circuit are fabricated in a multi-voltage level device using the same manufacturing process, the performance of the low-voltage circuit MOS transistors will decline.